# Certificateless Key Agreement

I'm studying Certificateless Cryptography and specifically I'm studying a scheme without pairings.

I read a lot of time this paper (https://www.sciencedirect.com/science/article/pii/S0898122112002490) but I am concerned about some issues in the paper.

Basically, during the key agreement procedure, when the user $$B$$ receives the first message, how can is possible for $$B$$ to know that the public key $$R_A$$ (along with $$ID_A$$) sent by the user $$A$$ is authentic (and belongs to $$A$$)? Furthermore, how can is possible demonstrate that is calculated starting from the Master Public Key of the Authority?

Ok, the key agreement works but who guarantee me that it is not a MITM attack?

Furthermore, how B retrieves $$P_A$$ to generate the key $$K_{BA}^2$$ if $$A$$ did not send it in the $$M_1$$?